1. Field of the Invention
The present invention relates to an optical lens system including a lens that has a tilt correction function and enjoys a decreased dimension in a sub-scanning direction without producing distortion or warpage of an effective portion of the lens that transmits a laser beam.
2. Discussion of Related Art
There has been known a laser printer including an optical scanning device that deflects, with a polygon mirror, a so-called “cylindrical” lens, etc., a laser beam representing an image to be formed, so that the deflected laser beam scans a photosensitive body in a certain direction, i.e., a main scanning direction. The cylindrical lens has a tilt correction function.
In the above-indicated optical scanning device, the laser beam is used to scan the photosensitive body, in the following manner: First, a laser-beam emitter emits the laser beam based on image data representing the image to be formed. Subsequently, the polygon mirror that is driven or rotated by a polygon-mirror drive motor deflects the laser beam in the main scanning direction. After an fθ lens transmits the deflected laser beam, the cylindrical lens corrects a tilt of the leaser beam in a sub-scanning direction perpendicular to both a direction in which the laser beam travels (hereinafter, this direction will be referred to as the “optical-axis direction”, if appropriate), and the main scanning direction. Subsequently, a reflect mirror such as a return mirror reflects the laser beam, toward the photosensitive body in the form of a rotatable drum, so that an electrostatic latent image corresponding to the image data is formed on an outer circumferential surface of the photosensitive drum.
It has been proposed to form the cylindrical lens employed by the above-indicated optical scanning device, of a plastic material such as a thermoplastic resin. An example of the cylindrical lens is disclosed by Japanese Patent Application Publication P2000-352679A1 and its corresponding U.S. patent application Publication 2002/0163571A1. This plastic cylindrical lens is formed by compression molding, i.e., first supplying the plastic material to molding dies, subsequently letting the molded plastic material to cool down to a sufficiently low temperature, and then removing the thus formed cylindrical lens from the molding dies.
An example 204 of the above-indicated conventional cylindrical lens is shown in FIGS. 6A and 6B. The conventional cylindrical lens 204 includes an effective lens portion 204a having a possibility that a laser beam is transmitted therethrough, and two rib portions 204b, 204c that are located on either side of the effective lens portion 204a in the sub-scanning direction and project outward from the same 204a in the optical-axis direction.
However, when the above-indicated cylindrical lens is cooled down after it is formed by the compression molding of the plastic material, a temperature difference is produced between a superficial or outer-surface portion, and a core portion, of the cylindrical lens, that is, the outer-surface portion is hardened before the core portion, and subsequently the core portion is hardened little by little. Thus, the rib portions 204b, 204c are hardened before the effective lens portion 204a, so that tensile forces may be produced in the effective lens portion 204a. The tensile forces may cause distortion or warpage of the effective lens portion 204a, and thereby deteriorate optical characteristics of the cylindrical lens 204.
To avoid the above-indicated problems of the conventional cylindrical lens 204, the lens 204 includes a wide margin between the effective lens portion 204a and each of the two rib portions 204b, 204c. 